When I forst wrote the cat1 test program, I interfaced to the keyboard with an interrupt. The AVR had a pin change interrupt. When a key is pressed, the keyboard has a valid bit ready on the falling edge, so having the pin change interrupt make an effecient way to read the keyboard. There was a lot of fussing around getting it to work with the video interrupt. Ultimately, I ended up dropping the interrupt idea, and instead polled the keyboard port in software on every scanline.
My Hackaday.io post on my previous troubles with the Keyboard interruptToday, I was looking for a better way. I googled 'AVR pin change flag without interrupts,' and came across a few claims like this saying the AVR will still set the pin change interrupt FLAG even with that particular interrupt disabled. I tried it in Atmel Studio's simulator, and it seems to be correct.
I still need to check the flag... PCIFR, every scanline. But this is much less work than I had before. I added code to read the port, compare it to the last read (which means storing the last state in a variable), and the clock cycles were adding up. It was threatening some pixels int he display: I needed to blank out a few pixels to make space for this. With the flag, I don't think I have to. (I will probably end up blanking a few pixels: both to create an invisible draw space for horizontal smooth scrolling, and to give pixels to the sound output engine.)
The plan now is to check the flag at the beginning of the scanline... if it needs to be serviced, the scanline will not be drawn. If I get basic keyboard working along with this video driver, I will move on to the next thing: probably the SD card.